Immunoassays for the detection of circulating tumor associated antigens and assays for the support of clinical trials have been developed. We are using these assays to examine sera and effusions from a variety of sources to study their utility to detect carcinoma. We used a second generation of MAbs against a tumor-associated antigen, TAG-72, in the development of a more efficient assay for TAG-72. This assay, designated CA 72-4 (using MAbs B72.3 and CC49, developed in collaboration with Centocor) was used to examine a panel of sera from patients with adenocarcinomas as well as benign diseases. Only 0.5% of 744 sera primarily obtained from blood donors had TAG-72 levels >10 U/ml. Similarly only 2.2% of 134 samples from patients with benign gastrointestinal diseases had TAG-72 levels >10 U/ml. Approximately 32% of 303 patients with gastrointestinal malignancies had serum TAG-72 >10 U/ml; approximately 44% of the patients with advanced disease had elevated TAG-72 levels. Patients with adenocarcinomas from a variety of other sites also had levels of TAG-72 above the reference value. The utility of the CA 72-4 assay was clearly demonstrated in the case of patients with gastric cancer; CEA was elevated in 37% of patients with Stage 3 and 4 disease, and 48% of the sera had elevated levels of TAG-72. No relationship was observed between the CEA and the CA 72-4 assays. Thus, the use of both assays to analyze these sera is clearly more advantageous than using the CEA or the CA 72-4 assay alone. The interaction in vivo of TAG-72 with radiolabeled MAbs was also studied to aid in understanding the pharmacology of MAbs used as radiopharmaceuticals. We found immune complex formation, even in the earliest sera drawn, as detected both by HPLC and SDS-PAGE analysis. Three different profiles, dependent on the serum TAG-72 level, were seen in the HPLC patterns. The presence of immune complexes did not appear to alter the clearance pattern of the 131I-MAb.